1. Field of the Invention
The present invention relates to drug-eluting medical devices; more particularly, this invention relates to support structure for medical devices. The support structure is used to support the medical device during processing of the medical device, in particular, when a coating is applied to the medical device.
2. Background of the Invention
Balloon expandable, or self-expanding stents are used to treat a variety of conditions, or prevent conditions from re-occurring within the body. For example, in percutaneous transluminal coronary angioplasty (PTCA), a balloon catheter is inserted through a brachial or femoral artery, positioned across a coronary artery occlusion, and inflated to compress the atherosclerotic plaque to open, by remodeling, the lumen of the coronary artery. The balloon is then deflated and withdrawn. Stents are used to prevent re-occurrence of conditions such as thrombosis and restenosis, which may occur several months after an angioplasty procedure is performed.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. Stent delivery refers to introducing and transporting the stent through an anatomical lumen to a desired treatment site, such as a lesion in a vessel. An anatomical lumen can be any cavity, duct, of a tubular organ such as a blood vessel, urinary tract, and bile duct. Stent deployment corresponds to expansion of the stent within the anatomical lumen at the region requiring treatment. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into an anatomical lumen, advancing the catheter in the anatomical lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen with the stent remaining at the treatment location.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon prior to insertion in an anatomical lumen. At the treatment site within the lumen, the stent is expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn from the stent and the lumen, leaving the stent at the treatment site. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath. When the stent is at the treatment site, the sheath may be withdrawn which allows the stent to self-expand.
Stents are often modified to provide drug delivery capabilities to further address thrombosis and restenosis. Stents may be coated with a polymeric carrier impregnated with a drug or therapeutic substance. A conventional method of coating includes applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend and applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. Depending on the application and volatility of the solvent, forced air drying may also be used to remove the solvent from the coating and arrive at a desired release rate for the impregnated therapeutic agent into the body.
It is known that some methods for coating stents do not produce an ideal, or defect-free coating over a surface of the stent where the therapeutic agent is intended to take effect. Coating defects may include non-uniform surface characteristics, such as bare spots and flaking. Coating defects can also serve as an initiation site for later-developed peeling or flaking that produces embolic debris. Rough surfaces generated by, and stagnant regions of blood flow produced nearby flaps or packets formed by coating defects can serve as a nidus for thrombus formation. Furthermore, coating defects produce variations in the intended amount, concentration, and release rate of the drug from the stent coating, further complicating or minimizing the effectiveness of therapeutic agents.
Methods for spray coating a stent with polymer-drug dissolved in a solvent include mounting the stent on a mandrel to support and rotate the stent while it is being sprayed with the drug-polymer composition. Examples of prior mandrel designs constructed for this purpose are disclosed in U.S. patent application Ser. Nos. 12/497,133; 12/027,947; 12/554,671 and 11/764,006 (U.S. Pat. No. 7,897,195); U.S. Pat. No. 7,572,336; and U.S. Pub. No. 2006/0035012.
In view of the foregoing, there is a continuing need for coating methods and systems that further minimize stent coating defects.